FIG. 2 is a structural drawing in section of an example of a cold-dielectric type superconducting cable having three twisted cores. FIG. 3 is a perspective view of a cable core.
The superconducting cable usually comprises twisted cable cores 101 contained in an interior thereof, a double insulation tube (a corrugated inner tube 103 and a corrugated outer tube 105) arranged at the outside of the cores 101, a protective covering outer sheath 106, and a thermal insulation 104 interposed between the both tubes 103, 105. The room between outer surfaces of the cable cores 101 and an inner surface of the corrugated inner tube 103 is a coolant (e.g. liquid nitrogen) flow channel 102.
Each cable core 101 comprises a hollow or solid former 107, a superconducting layer 108, a cold dielectric layer 109, a superconducting shield layer 110 and a protecting layer 111 formed of synthetic resin, which are arranged from the inside in this order. The dielectric layer 109 is usually in the form of kraft insulating paper or semisynthetic paper, including laminate insulating paper, such as polypropylene laminate paper (laminate insulating paper formed of polypropylene and kraft paper) (Brand name: PPLP), wound onto the superconducting layer.
FIG. 4 is a schematic diagram of a conventional superconducting cable line. FIG. 5 is an enlarged illustration of a part C of FIG. 4. The superconducting cable line 15 in which the superconducting cable constructed above is set is connected with pipes (a supply pipe 19′ and a return pipe 19) at both ends thereof to form a loop therebetween. Also, the superconducting cable line 15 is provided, at an intermediate portion thereof, with a cooling device 16 to cool coolant such as liquid nitrogen. The superconducting cable line 15 is formed by the superconducting cables being joined together through a joining portion 15B and is provided, at both ends thereof, with three-phase branch boxes 15A, 15C, to branch the cable cores contained in the superconducting cable into three phases. The cable cores are connected with branch pipes 21, respectively. The branch pipes 21 are connected to the supply pipe 19′ for supplying the coolant and the return pipe 19 for returning the coolant to the cooling device 16. The cooling device 16 is provided with a refrigerator 17 for cooling the coolant and a circulation pump 18 for circulating the coolant.
As shown in FIG. 5, the branch pipe 21 at the terminal end portion of the superconducting cable line comprises an outer tube 21A and an inner tube 21B between which a vacuum insulation layer 19B is disposed. Similarly, the return pipe 19 connected to the branch pipe 21 comprises an outer tube 19A and an inner tube 19C between which a vacuum insulation layer 19B is disposed.
A cooling method of this conventional superconducting cable line will be described with reference to the schematic diagram shown in FIG. 4.
When the superconducting cable line 15 is in its initial state of construction in which the superconducting cable is just set in the superconducting cable line 15, the coolant is not yet put in circulation through the flow channel 102 shown in FIG. 2 and the flow channel 102 is in the hollow state. When the superconducting cable line 15 is put into operation, the coolant is fed from the cooling device 16 to the flow channel 102. Outline arrows in FIG. 4 indicate a flowing direction of the coolant. As indicated by the outline arrows, during the operation of the superconducting cable line 15, the circulation cooling is repeatedly performed in a closed loop system of superconducting cable line 15→return pipe 19→cooling device 16 →supply pipe 19′→superconducting cable line 15. In this circulation cooling, the coolant works to cool the superconducting layer 108 and superconducting shield layer 110 of FIG. 3 and also contributes to electric insulation of the cable.
For some time after the beginning of cooling, the coolant cooling down the superconducting cable line 15 evaporates into gas to gradually lower the temperature of the cable line 15, while flowing along a longitudinal direction of the cable line 15. For example, when liquid nitrogen is used as the coolant, the liquid nitrogen gasifies for some time after the beginning of cooling, while flowing along the longitudinal direction of the cable line 15, to lower the temperature of the superconducting cable line of normal temperature down to the temperature of the liquid nitrogen (about −196° C.). The gas generated is discharged from a purged pipe 22 located at an intermediate portion of the return pipe 19 through a valve V4 opened.
As mentioned above, for some time after the beginning of cooling, the coolant in the superconducting cable line 15 is not impregnated into the dielectric layer 109 of the superconducting cable, so that the dielectric layer 109 is still in its initial state in which the insulating paper, such as the kraft insulating paper or the semisynthetic paper such as polypropylene laminate paper, is wound onto the superconducting layer. In the application of the insulating paper for the dielectric layer of an oil filled cable, the insulating paper is, in general, subjected to vacuum drying before applied to such a cable. This is because there is the possibility that any moisture contained in the insulating paper may accelerate deterioration of the electric properties of the cable. In the case of the superconducting cable, the dielectric layer can be evacuated when the thermal insulation pipe is evacuated in the manufacturing process. However, in the event that the insulating layer is opened to the atmosphere in a terminal-end treatment of the cable, moisture is entrained into a surface of the dielectric layer 109, resulting in that the surface of the dielectric layer 109 has a moisture content of approximately 5,000 ppm. Even after the cable is laid, an inner surface of the corrugated tube 103 and the entire cable core 101 of the cable are often put in the moisture absorption state.
When the cooling of the superconducting cable line is started in the condition that the surface of the dielectric layer is in the moisture entrained state, the entrained moistures are cooled and solidified by the coolant, leading to a possible problem that the ices or solidified moistures may be clogged at the terminal end portion of the cable line.
When the operation of the superconducting cable line 15 is started, the moistures from the dielectric layer of the cable core 101 and others are cooled by the coolant, so that the moistures cooled are condensed or solidified in a sherbet-ice-like form or a granular form. Then, the nitrogen gas and others evaporating and flowing in sequence through the cable line try to carry those solidified moistures toward the discharge port B and discharge them out together from the discharge port B.
It is usual that the return pipe 19 has an inner diameter as narrow as approximately 20 mm and also is turned at a right angle or at a nearly right angle. Due to this, the ices or the moistures solidified in a sherbet-ice-like form or a granular form are easily congested around the joint 21C (viewed in FIG. 5) between the branch pipe 21 and the return pipe 19 in the cable line at the discharge side thereof (at the right-hand side as viewed in FIG. 4). This congestion of the ices or the solidified moistures may cause clogging of the return pipe 19 or reduction of a section area of the clean inner tube 19C, to cause a possible failure to circulate the coolant and a possible failure to cool down the cable line.
It may be conceivable that the piping is heated from outside to melt the congested material, so as to remove it. But, even when the return pipe and the branch pipe are heated from the outside, since those pipes are thermally insulated by evacuation, the heat is not transferred to the inside of those pipes with ease, so it is very difficult to melt the congested material by heating the piping from the outside. Additionally, once the return pipe and the branch pipe are heated, it requires a long time to restore those pipes to their former state, thus causing great losses in time and economic aspect.
It is a principal object of the present invention to provide a cooling method of a superconducting cable line wherein moistures contained in a cable housed in the superconducting cable line after set in are removed from the cable before cooling, to prevent the moistures from being solidified, so as to prevent a coolant flow channel and piping, such as a return pipe, from being clogged with the solidified moistures.